(1) Field of the Invention
This invention relates to encapsulation of substrate based electronic packages using injection molding with a two piece mold and more particularly to the use of a barrier material formed over the gating area of the substrate which is later peeled away and discarded.
(2) Description of the Related Art
Injection molding using a two piece mold is often used for encapsulation of electronic devices. This requires the formation of a gating area on the substrate surface for removal of the mold runner after the encapsulation is complete and the encapsulant is cured. The gating area usually has a metal formed on the substrate such as palladium or gold. This places severe restrictions on the routing of circuit traces formed on the surface of the substrate.
U.S. Pat. No. 5,635,671 to Freyman et al. describes a degating region having a material formed thereon chosen such that the material in the degating region forms a weak bond with the encapsulant used. Freyman et al. describe the used of degating material such as gold so that wiring traces must be routed away from the degating region.
U.S. Pat. No. 5,311,402 to Kobayashi et al. describes an integrated circuit chip bonded to a circuit board with a cap for hermetically sealing the chip. The cap is bonded to the circuit board at the edges of an open end thereof and bonded to the underside or bottom of the chip.
U.S. Pat. No. 5,099,101 to Millerick et al. describes an automatic laser trimming apparatus for semiconductor integrated chip packages which performs deflashing and degating operations.
U.S. Pat. No. 4,954,308 to Yabe et al. describes a resin encapsulating method using upper and lower half molds.
Electronic circuit packages typically comprise a substrate with one or more integrated circuit elements attached. Molded packages are often used for the encapsulation of the integrated circuit element because they provide a reliable encapsulation at a reasonable cost. FIG. 1 shows a cross section view of such a package. The package has a substrate 10 with an integrated circuit element 12 attached to the first surface 26 of the substrate 10. Circuit traces 18 on the first surface 26 of the substrate communicate with circuit traces 22 on the second surface 27 of the substrate using via connections 20 through the substrate. Input/output balls 24 provide ball grid array type input/output connections for the package. The input/output balls can be a material such as solder, solder coated copper, or the like. The connections between the integrated circuit element 12 and the circuit traces 18 on the first surface 26 of the substrate 10 are provided by wire bonds 16.
The substrate based package is encapsulated with a molded encapsulant 14. In most cases the molded encapsulant 14 is a molded plastic. In packages of this type the molded encapsulant is usually formed using an injection molding process using a two piece mold. The two piece mold is preferred because of cost, but requires a gate region on the substrate as will be explained with reference to FIGS. 2-5.
A cross section of a part of a two piece mold is shown in FIG. 2. The mold has an upper part 30, a lower part 28, a cavity 34 in the upper part 30 of the mold and a recess 29 in the lower part 28 of the mold. The substrate 10 with the integrated circuit element 12 attached fits into the recess 29 of the lower part 28 of the mold. As can be seen in FIG. 2, the input/output balls have not been formed on the substrate at this point in the processing. A mold runner channel 32 is formed in the upper part of the mold 30 and forms a path for the uncured encapsulant to flow into the cavity 34 in the upper part 30 of the mold. FIG. 3 is a plan view of the upper part 30 of the mold, taken along line 3-3xe2x80x2 of FIG. 2, showing the cavity 34 and the mold runner channel 32.
During the encapsulation process uncured encapsulant is forced to flow from a source, not shown, through the mold runner channel 32 into the cavity 34, thereby filling the cavity. When the encapsulant is cured, encapsulant in the mold runner channel is also cured forming a mold runner 33, as shown in FIG. 4. This mold runner 33 must be removed after the encapsulant has cured. To accomplish the removal of the mold runner 33 a degating region 36 is typically formed on the first surface 26 of the substrate 10 at the substrate location which will be directly under the mold runner channel 32, see FIGS. 2 and 4. The degating region is formed of a material chosen such that the adhesive force between the encapsulant and the degating region material is less than the adhesive force between the encapsulant and the substrate. The degating region material is usually a metal such as gold or palladium.
With degating regions formed in this manner the circuit traces 18 on the first surface of the substrate and vias 20 between the first surface and second surface of the substrate must be routed to avoid the degating region 36, as shown in FIG. 5. The dashed lines in FIG. 5 show the location of the mold runner channel 32 and the perimeter 35 of the cavity. Degating regions of this type consume valuable surface area on the first surface of the substrate which could be used for circuit traces of vias. In addition mold compound material normally will flash outside the degating region and can cause problems.
It is a principle objective of this invention to provide a method of encapsulation of substrate based electronic devices using a gating region on a substrate which can be formed directly over circuit tracing.
It is another principle objective of this invention to provide a substrate, for substrate based electronic devices, wherein the substrate uses a gating region on a substrate which can be formed directly over circuit tracing.
These objectives are achieved by attaching a barrier material to the region of the substrate where the mold runner channel will be located when the package is encapsulated. The barrier material can be attached directly over circuit traces or via holes. The barrier material is chosen such that the adhesive force between the barrier material and the adhesive is less than the adhesive force of the cured encapsulant to the barrier material. After the encapsulation has been completed and the encapsulant cured the mold runner is removed thereby also removing the barrier material.